Liquid crystals have found use in a variety of electro-optical and display device applications, in particular those which require compact, energy-efficient, voltage-controlled light valves such as watch and calculator displays. These devices are based upon the dielectric alignment effects in nematic, cholesteric and smectic phases of the liquid crystal compound in which, by virtue of dielectric anisotropy, the average molecular long axis of the compound takes up a preferred orientation in an applied electric field. Since the coupling to an applied electric field by this mechanism is rather weak, the resultant electro-optical response time may be too slow for many potential applications.
Liquid crystal displays have a number of unique characteristics, including low voltage and low power of operation, which makes them perhaps the most promising of the non-emissive electro-optical display candidates available with today's technology. However, slow response and insufficient nonlinearity can impose limitations for many potential applications. The requirement for speed may become especially important in proportion to the number of elements which have to be addressed in a device. This may result in increasingly impractical production costs for the potential use of such devices in flat-panel displays for use in video terminals, oscilloscopes, radar and television screens.
It has been shown by N. A. Clark and S. T. Lagerwal in Appl. Phys. Lett. 36: 899 (1980) and in U.S. Pat. No. 4,367,924 that electro-optic effects with sub-microsecond switching speeds are achievable using the technology of ferroelectric liquid crystals (FLCs). Some display structures prepared using FLC materials, in addition to the high speed (about 1,000 times faster than currently used twisted nematic devices) reported by these investigators, also exhibit bistable, threshold sensitive switching, making them potential candidates for light modulation devices including matrix addressed light valves containing a large number of elements for passive displays or graphic and pictorial information, as well as for optical processing applications.
Smectic C liquid crystal phases composed of chiral, nonracemic molecules possess a spontaneous ferroelectric polarization, or macroscopic dipole moment, deriving from a dissymmetry in the orientation of molecular dipoles in the liquid crystal phases (Myer et al. (1975) J. Phys. (Les Ulis, Fr) 36:L-69). The ferroelectric polarization density is an intrinsic property of the material making up the phase and has a magnitude and sign for a given material under a given set of conditions. In ferroelectric liquid crystal display devices, like those of Clark and Lagerwal, appropriate application of an external electric field results in alignment of the molecules in the ferroelectric liquid crystal phase with the applied field. When the sign of the applied field is reversed, realignment or switching of the FLC molecules occurs. This switching can be employed for light modulation. Within a large range of electric field strengths, the switching speed (optical rise time) is inversely proportional to applied field strength and polarization or dipole density (P), and directly proportional to orientational viscosity. High switching speeds are then associated with FLC phases which possess high polarization density and low orientational viscosity.
A basic requirement for application of ferroelectric liquid crystals in such devices is the availability of chemically stable liquid crystal compounds or mixtures which exhibit ferroelectric phases (chiral smectic C) over a substantial temperature range about room temperature. In some cases, the ferroelectric liquid crystal compound itself will possess an enantiotropic or monotropic ferroelectric (chiral smectic C*) liquid crystal phase. Ferroelectric liquid crystal mixtures possessing chiral smectic C* phases with useful temperature ranges can also be obtained by admixture of chiral, nonracemic compounds, designated ferroelectric liquid crystal dopants into liquid crystal host material (which may or may not be composed of chiral molecules). Addition of the dopant can affect the ferroelectric polarization density and/or the viscosity of the C* phase and thereby affect the switching speed. Desirable FLC dopants are molecules which impart high ferroelectric polarization density to an FLC material without significantly increasing the orientational viscosity of the mixture.
Thermotropic liquid crystal molecules typically possess structures which combine a rigid core coupled with two relatively "floppy" tails (see Demus et al. (1974) Flussige Kristalle In Tabellen, VEB Deutscher Verlag fur Grundstoffindustrie, Lebzig for a compilation of the molecular structures of LC molecules). FLC materials have been prepared by the introduction of a stereocenter into one of the tails, thus introducing chirality. The first FLC compound to be characterized was DOBAMBC (Meyer et al., supra) which contains a 2-methylbutyl chiral tail. Pure DOBAMBC exhibits a smectic C* phase with a ferroelectric polarization of -3 nC/cm.sup.2. The structures and polarization of several known smectic C* materials, including several containing phenyl benzoate cores, have been summarized in Walba et al. (1986a) J. Amer. Chem. Soc. 108: 5210-5221, which also discusses a number of empirical correlations between molecular structure and FLC properties. This reference also reports the synthesis and properties of FLC compounds which contain nonracemic 2-alkoxy-1-propoxy tail units, derived from lactic acid, coupled to p-alkoxy phenyl benzoate cores. These compounds, also the subject of U.S. Pat. No. 4,556,727, possess monotropic smectic C* phases which display fast switching speeds at room temperature. It is also reported therein that certain eutectic mixtures containing these FLC compounds possess thermodynamically stable or entaniotropic smectic C* phases with high polarization density and fast electro-optical switching speeds. U.S. Pat. No. 4,556,727 discloses that the attachment of an enantiomerically enriched lactic acid derived tail unit to the para position of the phenyl group of a phenyl benzoate core unit will confer the desired properties of large ferroelectric dipole density and low orientational viscosity to a chirally asymmetric liquid cyrstal compound. The disclosure of that patent relates to ferroelectric smectic liquid crystals of the following general formula: ##STR2## wherein R is a lower alkyl group containing one to three carbon atoms and R' is an alkyl group containing nine to twelve carbon atoms.
In related work, Walba et al. (1986) J. Amer. Chem. Soc. 108: 7424-7425 and Walba and Vohra, U.S. Pat. No. 4,648,073 disclose ferroelectric (chiral) smectic liquid crystal compounds having an achiral core and chiral tail units derived from (2,3)-alkyloxiranemethanols which possess a high ferroelectric polarization density. The ferroelectric crystal materials reported have the following general formulas: ##STR3## where R is an alkyl of one to seven carbon atoms and R' is an alkyl of five to twelve carbon atoms.
Additional related work, Eidman and Walba, U.S. patent application Ser. No. 800,851, filed July 1, 1986, discloses chirally asymmetric liquid crystals possessing the phenyl benzoate core unit and 1-cyanoalkoxy chiral tails.
Also related is the subject matter of Walba and Razavi, U.S. patent application Ser. No. 911,096, filed Sept. 24, 1986, which discloses chirally asymmetric compounds possessing an achiral (phenyl benzoate) core unit with 1-fluoro or 1-chloroalkyl group chiral tail units having the general formula: ##STR4## wherein R is an alkyl group of three to twelve carbon atoms, R' is an alkyl of five to twelve carbon atoms, and X is a chlorine atom or a fluorine atom. These materials impart the property of high polarization density in admixtures which display an FLC phase and are useful as FLC dopants.
While several useful ferroelectric liquid crystal materials (both pure compounds and mixtures) have thus been reported, optimum response times have not been achieved (theoretical limit estimated as 10-50 nsec, Walba et al. (1986a), (supra). For this reason, new FLC materials particularly those having high polarization density and low viscosity are desirable, as are new FLC dopants which can impart desired properties to FLC materials.